# Collisions and Change in Momentum

#### jabbie3

1. The problem statement, all variables and given/known data

A ball of mass 4.7 kg moving with speed of 1.9 m/s in the x-direction hits a wall and bounces back with the same speed in the -x-direction. What is the change of momentum of the ball? And in what direction?

2. Relevant equations

p=mv

3. The attempt at a solution

P=mv
P=4.7*1.9
P=8.93

#### LowlyPion

Homework Helper
So what is the initial momentum?

What is the final momentum?

What is the difference?

#### jabbie3

The initial is 8.93 and the final is -8.93..so total momentum is 0? In what direction?

#### KD-jay

Try again using pf - pi to find the difference.

#### LowlyPion

Homework Helper
The initial is 8.93 and the final is -8.93..so total momentum is 0? In what direction?
Sorry, signs matter.

Initial momentum = P
Final momentum = -P

Final - initial = Δ = (-P) - P = -2P

Negative because Δx/Δt is toward -x.

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